1. Field of the Invention
This invention relates to a method for recording simultaneously inputted sound signals of a plurality of types such as stereo audio signals on a magnetic disk such as a floppy disk, and a method for reproducing such signals.
2. Description of the Prior Art
Devices are known for recording a video signal of a still picture on a magnetic disk, such as an electronic still camera, and one which uses a floppy disk, which is also called a "video floppy", for use in an electronic still camera, of approximately 47 mm in diameter and approximately 40 .mu.m in thickness which is the standard and practical use. The floppy disk of this type is rotated at a speed of 3,600 rpm to record 50 frames of still pictures on 50 tracks in a field recording mode, or 25 frames of still pictures on 50 tracks in a frame recording mode. The track has a width of 60 .mu.m, with a track pitch of 100 .mu.m, and a guard band of 40 .mu.m in width is provided between the tracks. Reproduced pictures may be displayed on a monitor such as a television screen, or may be printed out by a printer to obtain hard copies.
A time base compression is used for recording a sound signal on such a floppy disk, and a time base expansion is used for reproducing the signal from the floppy disk. Otherwise, since the floppy disk makes one turn in 1/60 second, only 1/60 second of signal can be recorded per track. By compressing the time base to 1/640, for example, a sound signal of about 10 seconds can be recorded on a track. The sound signal includes a variety of signals within an audio frequency band, such as a description of the still picture recorded, background music for the reproduced picture, and background sound when photographing by an electronic still camera.
These sound signals have been conventionally recorded in a monaural system.
FIG. 11 is a block diagram of a typical monaural sound signal recording system, and FIG. 12 is a block diagram of a monaural sound signal reproduction system.
Referring to FIG. 11, a sound signal 3 supplied from a microphone 1 to an input terminal 2 is passed through a low-pass filter (LPF) 4 and a noise reduction circuit (NR) 5, and then inputted to a time base compression device 6. The time base compression device 6 includes an A/D converter 7, a RAM (random access memory) 8, a D/A converter 9, an address counter 10, a write clock generator 11, and a read clock generator 12. The address counter 10 is connected to a recording start switch 13 and a PG detecting device 14 which outputs a PG signal 14a at a rate of one pulse per one turn of a video floppy 19 to detect the rotational phase of the video floppy 19. For a time compression factor of M, the frequency of a read clock signal 12a is set to M-times a frequency fs of a write clock signal 11a, that is, M.multidot.fs. A sound signal 5a from the noise reduction circuit 5 is converted to a digital sound signal 7a by the A/D converter 7 with a sampling frequency of fs. The digital sound signal 7a is stored in the RAM 8 after the recording start switch 13 is turned on and the address is advanced by the address counter 10 using a clock frequency of fs. Numeral 10a indicates an address signal. After the digital sound signal 7a is recorded in a predetermined area of the RAM 8, the address counter 10 advances the address in synchronization with a clock frequency of M.multidot.fs for reading the contents of the RAM 8. A digital signal 8a read from the RAM 8 is converted to an analog signal 9a by the D/A converter 9 using a clock frequency of M.multidot.fs. Thus, the obtained analog signal 9a is the sound signal 3 applied to the input terminal 2 having a time base that is compressed to 1/M.
The time-compressed analog signal 9a is emphasized in the high frequency range by a pre-emphasis circuit 15, modulated by a frequency modulator 16, and then recorded on an adequate track of the floppy disk 19 through a recording amplifier 17 and a magnetic head 18. Numeral 20 indicates a motor for rotating the floppy disk 19.
The time compression factor is determined by a frequency band which can be recorded on the floppy disk 19 and an upper limit frequency required for the sound signal 3 to be recorded. Electronic still cameras or the like can record a signal which is obtained by FM modulating a 6 MHz carrier with a sound signal of 3.2 MHz after compression, on a floppy disk which rotates at a speed of 3,600 rpm. Where the upper limit frequency is fv (KHz), the time compression factor M is given as M=3,200/fv. A time T of the sound signal to be recorded is given as T=M.multidot.1/60=3,200/60fv. Table 1 shows the values for fv=2.5 KHz, 5 KHz, and 10 KHz.
TABLE 1 ______________________________________ Re- Upper Re- Compres- Write Read cord- limit cording sion clock clock ing frequency time factor M fs(KHz) M.fs(MHz) mode fv (KHz) T (sec) (times) example example ______________________________________ 10 sec 5 about 10 640 10 6.4 5 sec 10 about 5 320 20 6.4 20 sec 2.5 about 20 1,280 5 6.4 ______________________________________
In the reproduction, referring to FIG. 12, when a reproduction switch 32 is turned on, a signal 21a is outputted from a magnetic head 21 to an amplifier 22, demodulated by a frequency demodulator 23, suppressed of its high frequency range by a de-emphasis circuit 24, and inputted to a time base expansion circuit 25. The time base expansion circuit 25 has an A/D converter 26, a RAM 27, a D/A converter 28, an address counter 29, a write clock generator 30, and a read clock generator 31. The address counter 29 is connected with the reproduction switch 32 and a PG detecting device 33 which outputs a PG signal 33a at a rate of one pulse per one turn. The frequency of a write clock signal 30a is the same as the frequency of the read clock signal 12a in the time base compression device 6 of the recording system shown in FIG. 11, that is M.multidot.fs. The frequency of a read clock signal 31a is the same as the frequency of the write clock signal 11a in the time base compression device 6, that is fs.
A time-compressed analog sound signal 24a obtained in the de-emphasis circuit 24 is converted to a digital signal 26a by the A/D converter 26 with a sampling frequency of M.multidot.fs. The digital signal 26a is stored in the RAM 27 with the address advanced by the address counter 29 using a clock frequency of M.multidot.fs which is synchronized with a PG signal 33a from the PG detecting device 33. Numeral 29a indicates an address signal. When the digital signal 26a is completely stored in a predetermined area of the RAM 27, the address counter 29 advances the address by a clock frequency of fs for reading the contents of the RAM 27. A digital signal 27a read from the RAM 7 is converted to an analog signal 28a by the D/A converter 28 using a clock frequency of fs. Thus, the obtained analog signal 28a is a sound frequency band signal having the original time base. The sound signal 28a is applied through a low-pass filter 34, a noise-reduction circuit 35, and an amplifier 36, to a speaker 37, and is outputted as sound.
As described above, in the prior art electronic still cameras or the like, sound signals have been recorded only in a monaural mode on a magnetic disk. There is a strong desire for recording sound in a stereo system. Further, there is a requirement for simultaneously recording two or more sound signals of any type, not restricted to stereo signals.
However, when a plurality of sound signals are recorded on a magnetic disk, the sound signals are required to be interchangeable for allowing reproduction by typical monaural reproduction devices.